1. Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to systems and methods for directing and correcting high-power beams of electromagnetic energy.
2. Description of the Related Art
Directed energy weapons and specifically high-energy laser (HEL) weapons are being considered for variety of military applications with respect to a variety of platforms, e.g., spaceborne, airborne and land based systems to name a few. These weapons generally involve the use of the laser or other source of a high-power electromagnetic radiation beam to track and destroy a target. To achieve mission objectives, directed energy weapons must be accurately steered and optimally focused. Steering involves line-of-sight control while focusing, with respect to HEL weapons, involves wavefront error correction.
In particular, high energy laser (HEL) beam control systems used on high performance tactical fighter aircraft require adaptive optics to correct for aero-optic distortions caused by turbulent boundary layers, flow separation zones, unsteady shocks and downstream wake structures in the immediate vicinity of the HEL beam director turret. Although the flowfield introduces severe time-varying wavefront errors, these optical path-length difference (OPD) errors occur only within the first tens of meters of propagation and the isoplanatic angle for such a thin, near-field aero-optic distortion is typically large, encompassing the entire extended target, not just the center. For this case, correcting the aberration along the line of sight to any part of the target (using a simple phase-only adaptive optic system) will effectively compensate the entire image of the target and provide a distortion-corrected path for the HEL beam to any aimpoint located on the target. The isoplanatic angle is the limiting angle across which optical rays emanating from the target follow essentially the same propagation path from the target to the beam control aperture. Stated differently, the optical phase fronts reaching the beam control aperture from two point sources at the target are just decorrelated when the points are separated by the isoplanatic angle.
As discussed above, advanced laser beam control systems typically use a two-illuminator system: one to track the hard body of a target and one for wavefront sensing. Active tracking typically requires a larger illuminated field due to the need to see the entire target, track features of the target (often located at the edges of the target), ascertain vulnerable aimpoints and maintain a stable line-of sight to the aimpoint. The wavefront sensing portion typically uses a beacon illuminator laser with a more narrow beam, which is directed by the beam control system to the aimpoint on the target. An adaptive optics system uses the return from the beacon illuminator to measure the wavefront error due to the flowfield (aero-optic) and the intervening atmosphere (atmospheric turbulence). This two-illuminator approach is necessary in long-range airborne theater missile defense and ground-based air defense scenarios due to the small isoplanatic angle relative to the size of the target hard body representative of these scenarios.
However, as is well-known in the art, inasmuch as high power, high performance lasers and optical systems are required to generate each of the two illumination beams, the two beam systems of the prior art are complicated and somewhat costly to implement.
Thus, a need exists in the art for a less complex and more affordable system or method for tracking a target, steering a high energy beam thereto, and correcting the optical phase distortions in the propagation path when a relatively large isoplanatic angle is anticipated.